Field of the Invention
The present invention relates to a light-emitting device including a semi-conductor light-emitting element such as a light-emitting diode element (hereinafter referred to as an LED element), more specifically to a light-emitting device having a long service life and capable of reducing deterioration caused by heat-generation.
Description of the Related Art
LED elements are widely used for backlights of color displays, illuminations or the like, because they have a long service life, excellent drive characteristic, high light-emitting efficiency and bright light-emitting color, and are compact.
In particular, if LED elements are used for, for example, flashes of cameras and for general lighting systems instead of conventional lamps such as fluorescent lamps or incandescent lamps, high-brightness is required for such usage. To achieve this, there has been proposed a light-emitting device including a plurality of diode packages each with a structure in which a plurality of LED elements are provided on a substrate and connected in parallel and with a structure of the plurality of diode packages connected each other in series (for reference, Japanese Patent Application Publication No. 2006-222412).
The proposed light-emitting device 100 includes a single elongated substrate 101 which includes a pair of first and second long sides and a pair of first and second short sides, and five diode packages L1, L2, L3, L4, and L5 which are arranged on the substrate 101 and connected in series, as shown in FIG. 15. Each of the diode packages L1 to L5 includes five LED elements 2a, 2b, 2c, 2d, and 2e which are disposed parallel to the pair of first and second short sides and connected in parallel to a pair of supply terminals 103a and 103b. Meanwhile, the diode package L1 is connected through a current-limiting resistor 102 one of the supply terminals 103a, and the diode package L5 is directly connected to another of the supply terminals 103b. 
The aforementioned conventional light-emitting device 100 operates as follows.
A light-emitting operation of the light-emitting device is performed by applying a voltage higher than a sum of forward voltages VF of the diode packages L1 to L5, that is to say, a voltage value in which a first forward voltage VF1 of the diode package L1, a second forward voltage VF2 of the diode package L2, a third forward voltage VF3 of the diode package L3, a fourth forward voltage VF4 of the diode package L4, and a fifth forward voltage VF5 of the diode package L5 are added, to the opposite supplying terminals 103a and 103b, and adjusting a current value at that time by the current-limiting resistor 102.
For this reason, it is necessary for the five LED elements 2a to 2e constituting each of the five diode packages L1 to L5 to select and uniform their forward voltages VF and preferable to set a range of variation in the forward voltages to 0.1V or less.
Consequently, even if one or two LED elements that constitute a diode package happen to be electrically disconnected, forward voltage can be supplied to the remaining LED elements in the diode package to light the remaining LED elements. As a result, electrical disconnection of a small number of LED elements has a less impact on operation of the entire illumination device, compared with a situation of total electrical disconnection of LED elements.
Because the number of the LED elements 2a to 2e in each of the diode package L1 to L5 is set to be the same, a circuit structure has a good balance and brightness can be held uniformly by equalizing current values flowing in the LED elements 2a to 2c. 
However, if the plurality of diode packages L1 to L5 are arranged on the single substrate 101 as mentioned above, a difference results in a state of heat generation of each diode package. That is to say, when the light-emitting device is operated to emit light, because all of LED elements 2a to 2e arranged in the diode packages L1 to L5 are connected in parallel with the corresponding electrodes 13a and 13b, the generally same current value flows in each LED element and generates heat in approximately the same way.
On the other hand, because the diode packages L1 and L5 positioned at elongated ends of the elongated substrate 101 can release heat outside from the elongated ends, this prevents these two diode packages disposed at elongated ends of the elongated substrate from buildup of the heat, whereas the diode packages L2 and L4 disposed away from the short sides of the elongated substrate or disposed inside of the diode packages L1 and L5 at elongated ends of the elongated substrate tend to keep accumulated buildup temperature, which are blocked by other diode packages L1 and L5. Furthermore, because the diode package L3 is positioned at a central portion of the substrate and sandwiched by the diode packages L1, L2, and the diode packages L3, L4, accumulated heat from the diode packages becomes highest among the diode packages L1 to L5.
FIG. 16 shows a temperature distribution corresponding to positions of the diode packages L1 to L5 on the substrate 101. The horizontal axis shows positions of the diode packages L1 to L5 arranged on the substrate 101, and the vertical axis shows temperatures, which correspond to positions of the diode packages L1 to L5.
As is clear from FIG. 16, the diode packages L1 and L5 arranged on the opposite sides of the substrate 101 are approximately a steady temperature Ts (° C.), whereas the diode package L3 arranged on the central portion of the substrate 101 has a higher temperature Tm (° C.) than the steady temperature. Meanwhile, each of the diode packages L2 and L4 has an intermediate temperature between the steady temperature Ts (° C.) and the higher temperature Tm (° C.). In this way, because the diode package L3 positioned on the central portion of the substrate 101 has a higher temperature than that of the diode packages L1, L2, L4 and L5 positioned at the other positions of the substrate, the LED elements 2a to 2e constituting the diode package L3 deteriorate and simultaneously occur changes in color tone, if the diode package L3 is used for a backlight of a display, there are problems of causing undesirable colors and a light-emitting device of a short life.